Conventionally, a seat back frame structure of a seat for a vehicle which is capable of reducing an impact load acting on a passenger has been adopted. The Japanese Patent Laid-open Publication HEI11-32865, for instance, discloses one example of such a seat back frame structure.
This seat back frame structure for a vehicle comprises a seat back frame structure integrally formed into a substantially C-shape, a tubular body connecting right and left longitudinal frames of the seat back frame structure, and a flattened tubular portion which is provided on this tubular body and is flattened so as to be bent due to the impact load acting on a passenger upon a rear collision of the vehicle.
According to the above seat back frame structure, a closed cross sectional structure (a box structure) is formed by connecting the right and left longitudinal frames in the widthwise direction through said tubular body and thus, a safety of a passenger upon the collision can be secured due to the fact that said flattened tubular portion functions as an energy absorbing portion which substantially restricts a destruction caused by the bending of the longitudinal frames upon the collision.
However, said conventional seat back frame structure has caused the following technical problems.
The strength or the rigidity of the seat back frame structure can be improved by the closed sectional structure (a box structure) of the seat back frame structure and the provision of the energy absorbing portion, however, the increase of the weight of the seat back frame structure is caused. This is contrary to a recent movement of a decrease of an amount of carbon dioxide emission by a decrease of an amount of the fuel being used.
In this connection, in order to limit such an increase of the weight, an example of adopting a high strength steel member such as an ultra high tension steel plate is disclosed in Japanese Patent Laid-open Publication 2008-67722. More specifically, left and right side frames with predetermined shapes into which the ultra high tension steel plate is formed by a press working is adopted.
However, such a high strength member is so expensive that the seat for a vehicle costs more, even if the increase of the weight can be restricted.
The seat back includes such a seat back frame structure which is mounted on the seat cushion so as to be inclined relative to the seat cushion, and a recliner is provided between the seat back frame structure and the seat cushion frame structure, so that in a case where the recliner is set to a locking situation, a bending moment acts on the seat cushion through the recliner when an impact load is exerted on the seat back. The seat cushion frame structure, like the seat back frame structure, includes a pair of side frames, each of which extends in the longitudinal direction of the vehicle, and each of the pair of side frames includes a C-shaped cross section so as to secure a section modulus in order to withstand such a bending moment.
However, like the seat back frame structure, in order to secure the strength or the rigidity, the weight of the seat cushion frame structure is inevitably increased, and its cost is also increased if the high tension member is adopted.